Exhaust after treatment system

ABSTRACT

A selective catalytic reduction system for reducing oxides of nitrogen in the exhaust gas flow of an internal combustion engine is disclosed. The system has a ceramic monolith disposed within said exhaust gas flow and includes exhaust flow passages extending therethrough. A high temperature selective catalytic reduction catalyst composition is applied to an inlet portion of the exhaust flow passages and a low temperature selective catalytic reduction catalyst composition applied to an outlet portion of the exhaust flow passages. The high and the low temperature catalytic reduction catalysts are operable to reduce oxides of nitrogen at high load and low load operation of the internal combustion engine.

BACKGROUND OF THE INVENTION

Emission treatment systems for internal combustion engines may include an oxidation catalyst upstream of a Selective Catalytic Reduction system that is useful for remediation of the nitrogen oxides (NO_(X)) in the exhaust stream. In diesel engines, a soot filter that is commonly referred to as a diesel particulate trap may also be included in the system for the removal of particulates from the exhaust gas.

Diesel engine exhaust is a heterogeneous mixture which contains not only gaseous emissions such as carbon monoxide (“CO”), unburned hydrocarbons (“HC”) and oxides of nitrogen (“NO_(X)”), but also condensed phase materials (liquids and solids) which constitute the particulate matter. Catalyst compositions, and substrates on which the catalysts are disposed may be provided in diesel engine exhaust systems to convert certain, or all of these exhaust constituents to non-regulated components. For example, diesel exhaust systems may include one or more of a diesel oxidation catalyst, a diesel particulate filter and a catalyst for the reduction of NO_(X).

One after treatment technology in use for high particulate matter reduction is the diesel particulate filter (“DPF”). There are several known filter structures that are effective in removing the particulate matter from diesel exhaust such as honeycomb wall flow filters, wound or packed fiber filters, open cell foams, sintered metal fibers, etc. The ceramic wall flow filters have experienced significant acceptance in automotive applications. The filter is a physical structure for removing particles from exhaust and, as such, accumulating particles will have the effect of increasing the backpressure on the engine. To address backpressure increases caused by the particulate accumulation the DPF is periodically regenerated. Regeneration involves the burning of accumulated particulates in what is typically a high temperature (>600 C), oxygen rich (lean) environment that may result in an increase in the levels of NO_(X) components in the exhaust gas stream. Similarly, in gasoline engines that employ lean burn technologies for increased fuel efficiency, a similar oxygen rich environment may also result in an increase in the levels of NO_(X) components in the exhaust gas.

A NO_(X) abatement technology that is being developed for automotive applications is Selective Catalytic Reduction (“SCR”) in which NO_(X) is reduced with ammonia (“NH₃”) to nitrogen (“N2”) over a catalyst that is typically comprised of base metals. For automotive applications, urea (typically present in an aqueous solution) is used as the source of the ammonia. SCR provides efficient conversion of NO_(X) as long as the exhaust temperature is within the active temperature range of the catalyst. An issue with known SCR catalysts is that high exhaust temperatures, such as are experienced during the DPF regeneration event in a diesel system or high load operation in a gasoline engine, may render many SCR catalyst compositions less catalytically effective while cooler, low load temperatures of engine exhaust may have a similar effect on other catalyst compositions.

Discrete substrates each containing catalysts to address specific components of the exhaust are available. However, it is desirable to reduce the overall size, complexity and cost of complete systems. One approach to achieve this goal is to coat the DPF with a catalyst composition which is effective for the conversion of the NO_(X) component of the exhaust stream and which is capable of efficient conversion at high and at low temperatures, across the entire range of operation of the DPF.

BRIEF DESCRIPTION OF THE INVENTION

In an exemplary embodiment, a selective catalytic reduction system for reducing oxides of nitrogen (“NO_(X)”) in the exhaust gas of an internal combustion engine comprises a ceramic monolith disposed within the exhaust gas and having longitudinally extending exhaust flow passages. A high temperature catalyst composition selected for high temperature catalytic reduction is applied to an inlet portion of the exhaust flow passages and a low temperature catalyst composition selected for low temperature catalytic reduction is applied to an outlet portion of the exhaust flow passages. The high temperature and the low temperature catalytic reduction catalysts operate to reduce oxides of nitrogen at high load and low load operation of the internal combustion engine.

These and other features and advantages of the invention will become more apparent to those skilled in the art from the detailed description of exemplary embodiments. The drawings that accompany the detailed description are described below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of an exhaust treatment system for an internal combustion engine;

FIG. 2 is an axial sectional view which schematically shows a ceramic wall flow monolith;

FIG. 3 is a NO_(X) reduction efficiency curve for the exhaust treatment system of FIG. 1;

FIG. 4 is an NH₃ oxidation curve for the exhaust treatment system of FIG. 1;

FIG. 5 is an embodiment of a catalyst loading curve for the exhaust treatment system of FIG. 1; and

FIG. 6 is another embodiment of a catalyst loading curve for the exhaust treatment system of FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 1, an exemplary embodiment of the invention is directed to an exhaust treatment system 10 for the reduction of regulated exhaust constituents of an internal combustion engine, such as diesel engine 12. The treatment system 10 includes an exhaust conduit 14 that transports the exhaust gas from the diesel engine 12 to the various exhaust treatment components of the exhaust treatment system. The exhaust components may include an oxidation catalyst 16 that is useful in treating unburned gaseous and non-volatile hydrocarbons and carbon monoxide, which are combusted to form carbon dioxide and water.

Downstream of the oxidation catalyst, a reductant may be injected as a spray via injector nozzle 18, into the exhaust gas flow 20, in exhaust conduit 14. Aqueous urea may be used as the ammonia precursor that may be mixed with air in the injector nozzle 18 to aid in dispersion of the injected spray. The exhaust stream containing the added ammonia is conveyed to a Selective Catalyst Reduction (“SCR”) device; in this case, Diesel Particulate Filter (“DPF”) 22. The DPF is operable to filter the exhaust gas to remove carbon and other particulates, and to reduce the oxides of nitrogen (“NO_(X)”) resident in the exhaust stream through the use of multiple SCR catalysts.

The DPF 22 may be constructed with a ceramic wall flow monolith 23, FIG. 2, which has a plurality of longitudinally extending passages 24 formed by longitudinally extending walls 26. The passages 24 include inlet passages 28 that have an open inlet end 30 and a closed outlet end 32, and outlet passages 4 that have a closed inlet end 36 and an open outlet end 38. Exhaust gas entering the DPF through the inlet end 30 of the inlet passages 28 is forced to migrate through the longitudinally extending walls 26 to the outlet passages 34. It is through this wall flow mechanism that the exhaust gas is filtered of carbon and other particulates. The filtered particulates 40 are collected on the walls 26 of the inlet passages 28. The accumulating particulates will have the effect of increasing the backpressure on the diesel engine 12. To address backpressure increases caused by the particulate accumulation the DPF is periodically regenerated. Regeneration involves burning of the accumulated particulates 40 in what is typically a high temperature (>600 C), oxygen rich (lean) environment that may result in an increase in the levels of the NO_(X) component in the exhaust gas stream.

In an exemplary embodiment of the emission treatment system 10, a first SCR catalyst composition 42 preferably contains a zeolite and base metal component such as Iron (“Fe”) which can operate efficiently to convert NO_(X) constituents in the exhaust gas flow 20 at the high temperatures experienced in the DPF 22 during regeneration (i.e. >600 C). Other suitable high temperature metals may include Cobalt (“Co”). The high temperature SCR catalyst composition 42 is applied to the walls of the inlet passages 28 of the ceramic wall flow monolith 23. A second SCR catalyst composition 44, also preferably containing a zeolite and base metal component such as Copper (“Cu”) which can operate efficiently to convert NO_(X) constituents in the exhaust gas flow 20 at low temperatures experienced in the DPF 22 during low load operation (i.e. <600 C), is similarly applied to the walls of the outlet passages 34 of the ceramic wall flow monolith 23. Other suitable low temperature metals may include Vanadium (“V”) and the like. FIGS. 3 and 4 illustrate the performance of the two SCR catalyst compositions 42 and 44 across the operating temperature range of the DPF 22. The NO_(X) reduction efficiency of the high temperature SCR catalyst composition 42 extends the conversion range of the device into the temperature range experienced during high load operation and regeneration of the DPF. The NO_(X) reduction efficiency of the low temperature SCR catalyst composition 44 extends the conversion range of the device into the temperature range experienced during low load or start up operation of the engine. In addition, and as illustrated in FIG. 4, during low temperature operation, the low temperature SCR catalyst composition 44 receives NH₃ that moves past the high temperature SCR catalyst composition 42. The low temperature SCR catalyst composition 44 utilizes the NH₃ to insure effective NO_(X) reduction. As such, the dual SCR catalyst combination allows the DPF to operate as an effective SCR system which is useful for remediation of the NO_(X) in the engine exhaust stream during high load operation or under DPF regeneration cycles as well as during low temperature, light load operation.

In an exemplary embodiment illustrated by the catalyst loading charts of FIGS. 5 and 6, application or loading of the high temperature and the low temperature SCR catalysts 42, 44 may be varied along the axial length of the filter resulting in a relatively uniform total catalyst loading 46 along the length of the ceramic monolith 23. In the embodiment illustrated in FIG. 5, the concentration of each catalyst varies in an axial direction so as to gradually increase or decrease as the case may be. The result is a uniform total loading of catalysts 42, 44 respectively. In the embodiment illustrated in FIG. 6, catalyst loading is constant in the axial direction but each catalyst primarily occupies a particular axial portion of the monolith 23, again resulting in a uniform total loading of catalysts 42, 44 respectively.

While the invention has been described with application to a ceramic wall flow monolith for the purpose of combining the DPF and the SCR catalyst devices, thereby eliminating a separate device from the exhaust system, it is contemplated that, in some circumstances separate devices may be dictated by the application. As indicated earlier, in gasoline engines that employ lean burn technologies for increased fuel efficiency, a similar oxygen rich environment may also result in an increase in the levels of NO_(X) components in the exhaust gas. While a DPF is typically not required with gasoline engines, the treatment of the exhaust gas flow from a lean burn gasoline engine may well benefit from a high temperature catalytic reduction catalyst composition applied to an inlet portion of a the exhaust flow passages of a flow-through (i.e. non-wall flow) monolith and a low temperature catalytic reduction catalyst composition applied to an outlet portion of the exhaust flow passages. As such, it is contemplated that the invention may also have application to straight flow ceramic monolith devices without straying from the scope of the invention.

While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and the scope of the appended claims. 

1. A selective catalytic reduction system for reducing oxides of nitrogen (“NO_(X)”) in the exhaust gas of an internal combustion engine comprising: a ceramic monolith disposed within the exhaust gas and having exhaust flow passages extending therethrough defined by longitudinally extending walls therebetween; a high temperature catalyst composition for high temperature catalytic reduction applied to an inlet portion of the exhaust flow passages; a low temperature catalyst composition for low temperature catalytic reduction applied to an outlet portion of the exhaust flow passages; and the high temperature and the low temperature catalytic reduction catalysts operable to reduce oxides of nitrogen at high load and low load operation of the internal combustion engine.
 2. The selective catalytic reduction system of claim 1, wherein the internal combustion engine is a diesel engine.
 3. The selective catalytic reduction system of claim 1, wherein the internal combustion engine is a gasoline engine.
 4. The selective catalytic reduction system of claim 2, wherein the ceramic monolith is a wall flow ceramic monolith, the longitudinally extending passages including inlet passages having an open inlet end and a closed outlet end and outlet passages having a closed inlet end and an open outlet end wherein the exhaust gas enters the wall flow monolith through the inlet end of the passages, and migrates through the longitudinally extending walls to the outlet passages.
 5. The selective catalyst reduction system of claim 4 wherein the high temperature catalyst composition for high temperature catalytic reduction is applied to the longitudinally extending walls of the inlet passages and the low temperature catalyst composition for low temperature catalytic reduction is applied to the longitudinally extending walls of the outlet passages.
 6. The selective catalyst reduction system of claim 1, the high temperature catalyst composition for high temperature catalytic reduction comprising a zeolite and a first base metal component and the low temperature catalyst composition for low temperature catalytic reduction comprising a zeolite and a second base metal component.
 7. The selective catalyst reduction system of claim 6, the first base metal component comprising iron.
 8. The selective catalyst reduction system of claim 6, the second base metal component comprising copper.
 9. The selective catalyst reduction system of claim 1 wherein the loading of the first and the second catalyst compositions may be varied along the axial length of the ceramic monolith resulting in a relatively uniform catalyst loading along the length of the monolith.
 10. A selective catalytic reduction system for reducing oxides of nitrogen (“NO_(X)”) in the exhaust gas flow of an internal combustion engine comprising: a ceramic wall flow monolith disposed within the exhaust gas flow; exhaust flow passages extending through the ceramic wall flow monolith and defined by longitudinally extending walls therebetween; the exhaust flow passages including inlet passages having an open inlet end and a closed outlet end and outlet passages having a closed inlet end and an open outlet end; wherein the exhaust gas enters the ceramic wall flow monolith through the inlet end of the passages inlet passages and migrates through the longitudinally extending walls to the outlet passages; a high temperature catalyst composition for high temperature catalytic reduction, applied to an inlet portion of the exhaust flow passages; a low temperature catalyst composition for low temperature catalytic reduction applied to an outlet portion of the exhaust flow passages; and the high temperature and the low temperature catalytic reduction catalysts operable to reduce oxides of nitrogen at high load and low load operation of the internal combustion engine.
 11. The selective catalytic reduction system of claim 10, wherein the internal combustion engine is a diesel engine.
 12. The selective catalytic reduction system of claim 10, wherein the internal combustion engine is a gasoline engine.
 13. The selective catalyst reduction system of claim 10, wherein the catalyst composition for high temperature catalytic reduction is applied to the longitudinally extending walls of the inlet passages and the catalyst composition for low temperature catalytic reduction is applied to the longitudinally extending walls of the outlet passages.
 14. The selective catalyst reduction system of claim 10, wherein the high temperature catalyst composition for high temperature catalytic reduction comprises a zeolite and a first base metal component and the low temperature catalyst composition for low temperature catalytic reduction comprises a zeolite and a second base metal component.
 15. The selective catalyst reduction system of claim 14, wherein the first base metal component comprises iron.
 16. The selective catalyst reduction system of claim 14, wherein the second base metal component comprises copper.
 15. The selective catalyst reduction system of claim 10 wherein the loading of the high temperature and the low temperature selective catalytic reduction catalyst compositions is varied along the axial length of the ceramic monolith resulting in a relatively uniform catalyst loading along the length of the monolith. 